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Thought Experiments in Teaching Free-Fall Weightlessness: a Critical Review and an Exploration of Mercury’S Behavior in “Falling Elevator”

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Thought Experiments in Teaching Free-Fall Weightlessness: a Critical Review and an Exploration of Mercury’S Behavior in “Falling Elevator” OPEN ACCESS EURASIA Journal of Mathematics Science and Technology Education ISSN: 1305-8223 (online) 1305-8215 (print) 2017 13(5):1283-1311 DOI 10.12973/eurasia.2017.00671a Thought Experiments in Teaching Free-Fall Weightlessness: A Critical Review and an Exploration of Mercury’s Behavior in “Falling Elevator” Jasmina Balukovic Druga Gimnazija, Sarajevo, BOSNIA and HERZEGOVINA Josip Slisko Benemérita Universidad Autónoma de Puebla, MEXICO Adrián Corona Cruz Benemérita Universidad Autónoma de Puebla, MEXICO Received 9 May2016 ▪ Revised 7 June 2016 ▪ Accepted 7 June 2016 ABSTRACT Different “thought experiments” dominate teaching approaches to weightlessness, reducing students’ opportunities for active physics learning, which should include observations, descriptions, explanations and predictions of real phenomena. Besides the controversy related to conceptual definitions of weight and weightlessness, we report another controversy regarding the position of the person that weighs herself or himself in a freely-falling elevator, a “thought experiment” commonly used for introducing the concept of weightlessness. Two XIX-century “thought experiments”, one from America and one from Russia, show that they have a long tradition in physics teaching. We explored experimentally a “thought experiment” that deals with the behavior of a mercury drop in a freely-falling elevator. Our experimental results show that the mercury drop neither took the expected spherical shape nor performed oscillatory motions predicted by theory. Teachers should encourage students to enrich active learning of weightlessness by thinking how to test experimentally the answers to some conceptual questions, a subclass of “thought experiments”. Keywords: Active learning, pedagogical content knowledge, scientific “habits of mind”, thought experiments, weightlessness in physics textbooks INTRODUCTION Recent advances in physics education were possible thanks to robust results resulting from many studies related to physics learning and teaching (McDemott & Redish, 1999; Thacker, 2003): (1) Students have “strong” or “soft” alternative conceptions about all studied physical concepts and phenomena whose presence can be detected in some © Authors. Terms and conditions of Creative Commons Attribution 4.0 International (CC BY 4.0) apply. Correspondence: Jasmina Balukovic, Druga gimnazija, Sarajevo, Bosnia and Herzegovina. [email protected] J. Balukovic et al. State of the literature Documental investigations on physics textbook treatments of weightlessness testify that there is no agreement between authors on a “scientific” definition of weight and weightlessness. For those who define weight “gravitationally”, the weightlessness of floating astronauts is “apparent”, while authors who define weight “operationally” consider weightlessness as real. Research on students’ conceptual understanding of weightlessness has been carried out almost exclusively with a focus on floating astronauts in spaceships. The most common alternative conception is that the astronauts float because there is no gravity. There are very few studies on how students conceptualize weightlessness phenomena in freely- falling systems that are feasible for classroom exploration. Contribution of this paper to the literature We provide evidence that physics textbooks (1) introduce the concept of weightlessness using a controversial “thought experiment” (person on scale in freely-falling elevator), and (2) formulate conceptual questions related to physical events in situations beyond the students’ practical experience. We argue that such an approach is adverse to students’ active learning of weightlessness phenomena that should be based on different types of feasible experiments. By experimental exploration of a “thought experiment”, we’ve shown that theoretical predictions should be taken always cum grano salis. We believe students are able to carry out similar experimental explorations of events described in conceptual questions. physics domain by established “concept inventories” (Hestenes, Wells & Swackhamer, 1992) or “conceptual surveys” (Maloney et al., 2001). (2) Those tests show that traditional lecture-based teaching is unable to improve many flaws in students’ conceptual understanding. (3) Designs of active physics learning are the most promising teaching strategies for feasible improvements of conceptual understanding (Meltzer & Thornton, 2012; Hake, 2007). In effective active-learning sequences, the experiments, actually performed by students or by teachers, play a crucial role. Thanks to them, students learn to know, without being told, what really happens in studied physical phenomena and in their further modifications. Depending on its place in students’ learning, an experiment can be (a) an “observation experiment” (students observe a phenomenon, determine its main features and propose different explanations for the phenomenon), (b) a “testing experiment” (students, alone or helped by their teacher, propose experiments to test their explanation scheme through logically-derived predictions), and (c) an “application experiment” (students propose new experiments to further test a provisionally accepted explanation) (Etkina et al., 2002; Etkina & van Heuvelen, 2007; Etkina, Gentile, & van Heuvelen, 2014). 1284 EURASIA J Math Sci and Tech Ed When students investigate physical phenomena in such a way they develop important “habits of mind” that make scientific work possible (Etkina et al., 2010) and are necessary for having more creative problem-solvers and decision-makers in today’s societies (Etkina & Planinšič, 2014). During the last three decades, philosophers of science were paying considerable attention to the role of “thought experiments” in development of scientific knowledge (Brown, 1991; Sorensen, 1992). Consequently, many physics and science educators were analyzing conceptual meaning and educational potentials of thought experiments, stressing their importance for introducing the “nature of science” into physics curricula and students’ learning of school physics (Helm, Gilbert & Watts, 1985; Reiner, 1998; Gilbert & Reiner, 2000; Reiner & Gilbert, 2000; Reiner & Gilbert, 2004; Reiner, 2006; Galili, 2009). After a careful analysis of many definitions found in the literature, Galili defines a “thought experiment” as “a set of hypothetico-deductive considerations regarding phenomena in the world of real objects, drawing on a certain theory (principle or view) that is used as a reference of validity” (Galili, 2009, p. 12). Velentzas and his collaborators carried out an important initial work on the implementation of some historical “thought experiments” (TE in what follows) in physics teaching and learning. They started by revising the presence and the forms of TEs in physics textbooks and trade books dealing with science popularization (Velentzas, Halkia & Skordoulis, 2007). Later they were able to show that a few historical TEs have a great potential to make possible learning of different, conceptually-demanding physics topics, such as elementary quantum theory (Velentzas & Halkia, 2011), satellite physics (Velentzas & Halkia, 2013a) and basic relativity (Velentzas & Halkia, 2013b). In Section 2, we present a polemic applicative TE, imagined by Pascal and exposed to a strong critique by Boyle. Possible sources of errors in elaboration of TEs are described, too. Section 3 provides a brief argument why public understanding of weightlessness is important. In Section 4 we report a conceptual controversy related to some fine details of the “thought experiment” with a freely-falling elevator that is commonly used for introducing the concept of weightlessness and formulation of some of conceptual questions. In Section 5, two old “thought experiments”, dealing with the unlikely behavior of falling liquids, are briefly described and commented. Section 6 brings an account of different experiment recreations of an “applicative TE” related to the behavior of a mercury drop in a free-falling elevator. In the closing section, we formulate some implications of our results for better teaching and learning of free-fall weightlessness. AN INSTRUCTIVE APPLICATIVE THOUGHT EXPERIMENT: BOYLE VS. PASCAL Important and far-reaching “thought experiments”, that strongly attracted the attention of philosophers of science, are relatively rare because they mark “death of old theories” or “birth of new theories”. Brown (1991) calls them “destructive TE” or “constructive TE”. More frequent in daily scientific work are modest “applicative TE” in which scientists explore 1285 J. Balukovic et al. conceptually and mathematically what should happen in a particular situation if an established theoretical model is applicable. Similarly, physics textbook authors and teachers commonly ask students to try to carry out mini “applicative TEs” by posing “conceptual questions”. Students’ tasks are to provide justified qualitative predictions about the course of physical events in different situations, from slightly modified ones that students know to those that are completely new to them. The popularity of these questions got a strong boost by the books of Hewitt (1971, 1993), Epstein and Hewitt (1979), and Epstein (2002), skillful and convincing advocates of the pedagogical credo: Conceptual understanding should be practiced and learned before numeric and algebraic manipulations. In the article, based on his Millikan lecture (Hewitt, 1983), Hewitt stressed: Physics is easy to teach mathematically, but we make a mistake
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